1. Field of the Invention
The present invention relates to a separator for a fuel cell stack, which is capable of facilitating flow of a fluid by controlling the difference in an internal pressure of a stack, and a fuel cell stack using the same.
2. Discussion of Related Art
A fuel cell is a generator system capable of directly converting a chemical reaction, which involves a fuel such as hydrogen, into an electrical energy. Such a fuel cell may be used for a small power source of notebook computers and portable electronic equipment, a power source for housings, a portable power source used in yachts or camping sites, a power source for electric vehicles, generators in large buildings, etc. As a basic structure of the fuel cell, a unit cell (hereinafter, referred to as a “cell”) generally outputs a voltage of 0.4 to 0.9 V. Accordingly, the most fuel cells are manufactured in a stacked structure where a plurality of cells is electrically coupled in parallel so as to obtain a desired voltage.
The fuel cell stack may be manufactured in a structure where a plurality of electrolyte membranes, membrane electrode assemblies and separators are stacked. The membrane electrode assembly (MEA) includes an anode and a cathode that are coupled respectively to both sides of an electrolyte membrane. And, the separators are disposed in both sides of the membrane electrode assembly, and functions to distribute and supply a fuel and/or an oxidant to the membrane electrode assembly. Such a stack structure is widely used since it is easily manufactured and has excellent performance.
Meanwhile, in a fuel cell stack that uses a liquid fuel, the liquid fuel flows into a separator through an inlet and flows out of the separator through an outlet during a normal operation of the fuel cell stack. However, gas is generated though the electrochemical reactions of the fuel during the operation of the fuel cell stack, and the volume of the gas rapidly increases causing increase of pressure of the gas. In this case, pressure of an outlet, through which the liquid fuel flows out of the separator, can be higher than pressure of an inlet, through which the liquid fuel flows into the separator. Therefore, the liquid fuel can flow backwards into the inlet, which is called a back flow phenomenon. In this case, the fuel may be non-uniformly supplied to each of the cell in the stack, and performances of the fuel cell may also deteriorate.
In order to solve the above problems, there is an alternative method for installing a first manifold and a second manifold in a manner that a cross-sectional area of the first manifold is smaller than that of the second manifold by modifying a structure of a manifold. However, this alternative method has a problem that a fuel may not be sufficiently supplied to cells disposed in the innermost of the first manifold due to the narrower cross-sectional area of the first manifold. Accordingly, it is difficult to apply this alternative method to a real fuel cell system, and there is also no effect to prevent a back flow of fluids.